1. Technical Field
The present disclosure relates to a lithium ion secondary battery.
2. Description of the Related Art
Typically, as the positive electrode material (positive electrode active material) for lithium ion secondary batteries, laminated compounds such as LiCoO2 and LiNi1/3Mn1/3Co1/3O2, and spinel compounds such as LiMn2O4 have been used. Recently, attentions are being focused on compounds having an olivine-type structure, such as represented by LiFePO4. Positive electrode materials having olivine structures are known to have high thermal stability at elevated temperature and high safety levels. However, a lithium ion secondary battery using LiFePO4 has low charging/discharging voltages of approximately 3.5 V, with an associated disadvantage of low energy density. To address this, LiCoPO4 and LiNiPO4 and the like have been proposed as phosphate-based positive electrode materials with which high charging/discharging voltages can be achieved.
However, even with the lithium ion secondary battery using such positive electrode materials, sufficient capacity has not been obtained. Among the phosphate-based positive electrode materials, vanadium phosphate with structures of LiVOPO4 and Li3V2(PO4)3 is known as a compound capable of achieving charging/discharging voltages on the order of 4 V. When the compounds are used as the positive electrode active material, there has been the problem of insufficient rapid charging characteristics at low temperatures. With regard to the positive electrode material, vanadium phosphate is described in JP-A-2003-68304 and JP-A-2009-231206. The literature, however, fails to describe the problem of rapid charging at low temperatures.